If you patched your Windows computers in 2010 against the LNK exploit used by Stuxnet and thought you were safe, researchers from Hewlett-Packard have some bad news for you: Microsoft’s fix was flawed.

In January, researcher Michael Heerklotz reported privately to HP’s Zero Day Initiative (ZDI) that the LNK patch released by Microsoft over four years ago can be bypassed.

This means that over the past four years attackers could have reverse-engineered Microsoft’s fix to create new LNK exploits that could infect Windows computers when USB storage devices got plugged into them. However, there’s no information yet to suggest this has happened.

The original attack, which exploited a vulnerability in how Windows displayed icons for shortcut (LNK) files, was used to spread Stuxnet, a computer worm that sabotaged uranium enrichment centrifuges at Iran’s nuclear facility in Natanz.

Stuxnet, which is believed to have been created by the U.S. and Israel, was discovered in June 2010 after it spread beyond its intended target and ended up infecting tens of thousands of computers around the world. The LNK vulnerability, tracked as CVE-2010-2568, was one of several zero-day, or previously unknown, flaws that Stuxnet exploited. Microsoft patched the flaw in August that same year as part of a security bulletin called MS10-046.

“To prevent this attack, Microsoft put in an explicit whitelist check with MS10-046, released in early August 2010,” the HP researchers said in a blog post Tuesday. “Once that patch was applied, in theory only approved .CPL files should have been able to be used to load non-standard icons for links.”

“The patch failed,” they said. “And for more than four years, all Windows systems have been vulnerable to exactly the same attack that Stuxnet used for initial deployment.”

ZDI reported the LNK patch bypass found by Heerklotz to Microsoft, which treated it as a new vulnerability (CVE-2015-0096) and fixed it Tuesday as part of MS15-020. The ZDI researchers plan to examine the new update to see if there are any other possible bypasses.

However, applying the workaround published by Microsoft in 2010, which involves using the registry editor to manually disable the display of icons for shortcut files, will protect against the latest flaw too, they said.

As revealed by a Kaspersky Lab report in August 2014, exploitation of the original CVE-2010-2568 vulnerability remained widespread even after the Microsoft patch in 2010, primarily because the exploit was integrated in more common threats like the Sality worm. From July 2010 to May 2014, Kaspersky Lab detected over 50 million instances of the CVE-2010-2568 exploit on more than 19 million computers worldwide.